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Best Size For Solar Charge Controller

A solar charge controller plays a vital role in any solar power system. Essentially, the charge controller is the regulator that limits the rate of current that flows to and from the system's battery bank. By controlling the flow of energy from your solar panel array, the solar charge controller can prevent overcharging issues and reverse the current flow when the solar panels are not generating electricity.

Despite the fact that the solar charge controller plays such an important role, few people know how to determine the correct unit for their solar power kit accurately.

You are in the market for a solar charge controller, but you don't what size solar charge controller. You want one that will be large enough to handle all your power needs but not too large. Read on for more information about sizing your solar charge controller based on how much power you need. 

What Are Solar Charge Controllers?

It is a device that, most importantly, properly charges a battery or battery bank, enabling as long a life as possible.

A solar charge controller is designed to receive power from a solar panel or array of panels and use it to charge a battery or bank of batteries.

The charge process is performed in a 'smart' or 'intelligent way. This involves varying charge stages, voltages, and current — depending on the type of battery and other factors.

Not to be confused with an inverter, a charge controller is specifically designed to charge batteries in a controlled fashion.

The charge controller in your solar installation sits between the energy source (solar panels) and storage (batteries). Charge controllers prevent your batteries from being overcharged by limiting the amount and rate of charge to your batteries. They also prevent battery drainage by shutting down the system if stored power falls below 50 per cent capacity and charging the batteries at the correct voltage level. This helps preserve the life and health of the batteries.

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Solar Charge Controller Functions

The function of a charge controller is to regulate the charge going into your battery bank from your solar panel array and prevent overcharging and reverse current flow at night.

It does this by using a transistor to shunt the PV charging circuit. This means, if your battery is full, it stops the charging, and if your battery is reaching an unhealthy discharge point, it stops the discharging.

By using a PV charge controller, you minimize the use of utility power and maximize the chances of your batteries and other photovoltaic components lasting longer, thus increasing your entire solar system's life expectancy and efficiency.

More sophisticated solar charge controllers make sure the battery is charged by utilizing pulse width modulation (PWM) or maximum power point tracking (MPPT).

By inputting pre-set high and low voltage cut off settings, you can help keep your batteries healthy and efficient automatically.

Why Do You Need a Solar Charge Controller?

In simple terms, a solar charge controller manages the power flowing into the battery bank from a solar array. They protect the expensive battery bank in a solar power system by preventing overcharging during daylight hours. They also prevent power from flowing backwards to the solar panels at night, which reduces the impact of phantom battery drain.

While some solar charge controllers have additional capabilities, like load control, managing power flow is their primary function. If you have invested in a battery bank for solar to store the energy, your system will require a solar charge controller.

What Are The Two Types Of Solar Charge Controllers?

Solar charge controllers are available in two separate configurations – PWM and MPPT. Understanding the differences between the two will help you determine the best option for the particular needs of your solar power system.

The difference between these two types of controllers is that the PWM is not as efficient as the MPPT. The MPPT is the most common these days and can gain you up to 30% more power than the PWM controllers. The MPPT controllers also allow the strings of panels to be connected in series for higher voltages, keeping the amperage lower and the wire size smaller, especially for long-wire runs to the PV array.     

When picking a charge controller, there are a few steps that you must follow to make sure that you get the right controller for the job. The best thing that you can do is use the manufacturer's sizing tools offered on their websites. The other option is to give the manufacturer a call — their salespeople will usually be happy to help you to pick the best controller.

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Pulse Width Modulation Charge Controller (PWM):

PWM solar charge controllers tend to be less expensive, as they feature less advanced technology. Essentially, a PWM acts as a simple switch, which turns on and off at set parameters to prevent overcharging issues.

Using a PWM, the solar panel, or panels, need to be at the same voltage as the battery, as you connect them (more solar panel connectors types) directly to the battery with just a switch separating them. Since the type of battery bank you are using will determine the output the solar panels can generate, there is a considerable loss factor, usually in the range of 30%. Find an in-depth guide on the solar battery bank

Maximum Power Point Tracking Charge Controller (MPPT):

MPPT charge controllers are considered 'smart' units, as they feature a built-in computer, which makes them programmable and much more adaptable than PWM controllers. They function by measuring the voltage of the panels and down-convert it to match the battery's voltage.

Since the power flowing into the battery from the charge controller is dropped to match the battery bank, the current can be raised, which means more of the available power flows from the panels to the battery. Basically, you can use a higher voltage solar array than the battery's voltage, which opens up a wide range of options and allows you to make your solar power system far more effective and even scale it up.

What Size Charge Controller Do I Need?

Take the number of panels x watts to get the total watts of the solar array. You then divide it by the voltage of your battery bank to get amps, add 25% to allow for cold temperatures and, as always, round up. Example: 2 - 140 watt solar panels in series = 280 watts / 12 VDC battery bank + 25% = 29.18 amps. You would choose a 30 amp, 12 VDC charger in this example. Another example would be 4 - 250 watt solar panels = 1,000 watts / 24V battery bank = 41.7 amps + 25% = 52.09 rounded up = 60 amp controller. Note; Solar charge controllers are rated and sized by the solar panel array current and system voltage. These examples are simple calculations for small systems. Suppose you have a larger system with multiple strings you are considering. In that case, you should look at our Off-Grid Living page for some pre-configured systems that include the right sized charge controller or consult the charge controller manufactures string calculator.

Sizing charge controllers is pretty simple, really. PV charge controllers are rated and sized depending on your solar array's current (amps) and the solar systems voltage (push).

Therefore solar charge controller sizing basically involves "getting a charge controller big enough to handle the amount of power and current produced by your solar energy system".

The most common PV charge controllers come in 12, 24 and 48 volts. Amperage ratings can be between 1-60 amps and voltage ratings from 6-60 volts.

So if your solar system's volts were 12 and your amps were 14, you would need a solar charge controller that had at least 14 amps.

However, due to factors such as light reflection, sporadic increased current levels can occur, so you need to factor in an additional 25% bringing the minimum amps that our PV charger controller must have to 17.5 amps.

So we'll need a 12 volt, 20 amp charge controller (rounded up).

It won't hurt anything if your charge controller's amps are higher; in fact, it's a good idea just in case you increase the size of your solar energy system in the future.

Add up the total watts of solar panels and divide by either 14.4 for 12-volt systems, 28.8 for 24 volts or 58.8 for 48-volt battery banks. This will give you maximum output amps from the controller. If you don't want to waste output in heat, size the controller at around two-thirds the rated output of the controller.

  • 20 amps are most efficient for a 30-amp controller
    • 10 amp to 30 amp Solar Charge Controllers
  • 45 amps are most efficient for a 60-amp controller
    • 31 amp to 60 amp Solar Charge Controllers
  • 65 amps are most efficient for an 80-amp controller
    • 61 amp to 80 amp Solar Charge Controllers
  • 80 amps are most efficient for a 100-amp controller
    • 81 amp and higher Solar Charge Controllers

Using these guidelines, the system won't run hot or waste precious energy. In addition, the fan won't fail prematurely, which is important because if that happens, the controller could release smoke that will be very tough to get back in.

Sizing a PWM Charge Controller:

Since they cannot limit their current output, you need to match and preferably exceed the current of your solar panels. Therefore, when searching for a PWM controller, look for the amperage and voltage rating and make sure these numbers exceed your solar array's rating and that of your battery.

After you have identified the voltage of your system and found a charge controller that matches or exceeds that value, you need to look at your battery's rated current. This is where we recommend choosing a charge controller that can exceed the amp rating of your solar array, as it can spike. A good practice is to exceed the amp rating by 25%, which means multiplying the amp rating of your solar panels by 1.25 and finding a charge controller with an amp rating that exceeds that number.

If finding a charge controller that matches and exceeds the nominal voltage of your system, the rated current of your battery, and the maximum solar input seems like a complicated task, do not worry. It is actually fairly straightforward once you have your numbers. However, it is worth noting that the entire process is easier if you opt for an MPPT charge controller.

Sizing an MPPT Charge Controller:

As explained above, MPPT charge controllers regulate output, so they do not need to be matched to the maximum output of the solar array. However, you will still want your MPPT charge controller to exceed the current potential of your solar array so that it can maximize the efficiency of your system.

Since MPPT controllers can lower voltage to match the battery bank's voltage and increase current to make up for the loss of power, they are far more adaptable.

You will still want to make sure that your charge controller can allow your system to run as efficiently as possible, so divide the system's total wattage by the lower voltage value between the solar array and battery. Follow Watts/Volts to determine the amps. So, if you were running a 900W solar array with 48V and your battery's voltage was 24V, you would divide 900W/24V to get a value of 37.5A. Adding an additional 25% for potential current spikes would give you a total of 46.9A.

This would mean choosing an MPPT charge controller that could handle at least 24V and 50A.

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Can You Use More Than One Charge Controller?

You can use multiple charge controllers with one battery bank in situations where a single charge controller is not large enough to handle the output of your solar panel array. In fact, for MPPT charge controllers, this can be the best way to connect your system as arrays have different maximum power points. Therefore, having two controllers can optimize the total power output.

However, we do recommend using the same type of charge controller if you are using more than one. So if you have one MPPT charge controller, all of your charge controllers should be MPPT. Additionally, you'll want to make sure all your controllers have the same battery setting input.

Frequently Asked Questions About Solar Charging

You take the total watts of the solar array divided by the voltage of the battery bank. That will give you the output current of the charge controller. For example, a 1000W solar array ÷ 24V battery bank = 41.6A. The rating of the charge controller should be at least 40A.

By the rule of thumb, a 100-watt solar panel inputs 30 amp-hours per day into your batteries. So you would need 1.33 100 watt panels or one 133 watt panel to match your solar power needs. You can't buy a third of a panel, nor can you buy a 133-watt panel. But you can buy a 160-watt panel.

For example, if the solar panel is 200-watt and you want to charge a 12-volt battery, you would divide 200 by 12 to get 16.6 amps. Charge controllers are usually rated in multiples of 30 amps, so you'd get a charge controller rated for 30 amps. 

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